A continued effort will be made to demonstrate mutagenesis in Syrian hamster embryo cell cultures. This may include transplacental mutagenesis and the choice of another genetic marker. The assay must then be placed on a quantitative basis to be able to compare mutant frequencies and study enhancement conditions. The Chinese hamster cell line will also be used for further studies of the mutation process itself and the mutagenic efficiencies of several activated carcinogens. The study should also be extended to existing man-mouse cell hybrids to demonstrate mutagenesis of specific human chromosomes.